German Offenlegungsschrift 3,433,926 (Mar. 27, 1986) to Streit et al, discloses a single bath reductive and oxidative bleaching process, in which the reductive bleaching with thiourea dioxide precedes an oxidative hydrogen peroxide bleaching, whereas in the processes of the present invention the reductive bleaching is subsequent to the oxidative bleaching and dyeing may be carried out in the same bath. Japanese patent 51-64082 (Jun. 3, 1976) is drawn to a process in which hydrogen peroxide and thiourea are mixed at-the start of the bleaching processes (i.e., bleaching with a single mixture which contains both hydrogen peroxide and thiourea), while by contrast the instant invention utilizes separate steps of oxidative bleaching followed by reductive bleaching and dyeing. It has unexpectedly and surprisingly been discovered that the processes of the present invention provide greatly improved results (including, a higher Whiteness Index, lower Yellowness Index, lower degree of damage, and improved dyeing) as compared to the results achieved by separate dyeing following either of the aforementioned prior art bleaching processes.
Scoured wool varies in shade from the light cream of wools considered to have good color to discolored urine-stained wools and the near blacks of heavily pigmented wool. Since even the background natural color of wool interferes with dyeing to specific pastel or bright colors, bleaching is practiced on all of these wools for such purposes. Therefore, in order to achieve bright shades, one has to either bleach wool prior to dyeing or incorporate a bleaching agent sometime during the dyeing cycle. The former process calls for a lengthy treatment, since wool has to be bleached and free of any residual bleaching agent before it may be successfully dyed. The latter process, though sounding promising, must take into account the sensitivity of some dyes toward bleaching agents, hence limiting the number of dyes that may be used. Two main factors will be operating in the dyeing of bleached fibers. These are the swelling of fibers, which results from disulfide scission caused by bleaching treatments, and the creation of negative charges on the surface, which may hinder dye adsorption and diffusion. As these factors work against each other, a compromise has to be found where there is sufficient swelling caused by bleaching but at the same time considerably less negative charge created at the surface. This should be achievable by reducing the concentration of bleaching reagents and limiting the severity of the bleaching conditions so that the factors above are maintained within limits to allow an acceptable dyed product. It is also known that the higher the concentration of bleaching agents and the more severe the bleaching conditions are, the greater the whiteness. Therefore a balance must be achieved between optimum whiteness and good quality dyeing. Lemin et al (Some physicochemical properties of damaged wools, J. Soc. Dyers Colour. 62, 129-144 (1946)) reported that hydrogen peroxide-bleached wool dyed to a weaker shade than did unbleached wool, because both dye affinity and the number of dye sites available were reduced from unbleached wool. Duffield, "Dyeing for machine washable fabrics", Proceedings of Wool in the Eighties Seminar, Wakefield, Mass., April 1984, pp. 173-174 reported the "Tubotex PC.sup.2 " (mixture of sodium silicate and caustic soda with pH 9.0-10.0) prebleaching process; bright pastel shades from subsequent dyeing are fostered by this rapid, low temperature alkaline hydrogen peroxide bleaching (1 hour; 40.degree. C.; pH 9.0-10.0 using sodium silicate and caustic soda). This process has been used in several bleaching houses with improved results in terms of whiteness and residual fiber damage, owing to the low temperature of the bleaching process.
There are numerous scientific papers and patents on the simultaneous dyeing and bleaching of fibers (see e.g. Anonymous (to Ciba-Geigy Co.), Simultaneous dyeing and bleaching fibres, Swiss Patent 63-14,624 (Nov. 29, 1963); Arsov, A., Kunchev, E., Serafimov, S., Single bath bleaching and dyeing of wool by acid azo dyes, Tekst. Prom. (Sofia), 19(10), 29-32 (1970), CA 75(4)22318k; BASF A.G., Technical Leaflet M5756e (1981); Duffield, P. A., Review of Wool Bleaching, IWS Technical Information Bulletin, 31-33 (1986); Nikolova, A., Arsov, A., Kunchev, E., Medyalkova, K., Single bath dyeing and bleaching of mixed wool-artificial fibre yarn, Tekst. Prom. (Sofia), 21(4), 193-196 (1972), CA 81(14)79197y; Schmidt, O., Simultaneous dyeing and bleaching of proteinaceous fibrous material, U.S. Pat. No. 3,551,087 (Dec. 29, 1970); Schmidt, O., Simultaneous dyeing and bleaching of wool, German Patent 1,469,609 (2/26/1970); Schmidt, O., Procedure to bleach and to dye protein-containing fibres in a single bath, Belgian Patent 672,398 (May 16, 1966); Senner, P., Ulmer, D., Renner, J., Possibilities of single bath dyeing and bleaching of wool, Z. Gesamte Textilind., 68(10), 763-767 (1966), 68(11), 858-863 (1966); Uehara, N., Dyeing and bleaching of fibers, Japanese Patent 73 20,975 (Mar. 15, 1973); and Uehara, N., Single bath bleaching and dyeing of textiles, Japanese Patent 73 61,782 (Aug. 29, 1973)). Schmidt supra, reported simultaneous dyeing and bleaching of proteinaceous and other fibers, where the fibers are treated at elevated temperatures in a non-alkaline aqueous liquor that contains (a) an acid dye capable of dyeing wool and resistant to peroxides, and (b) performic acid in the form of the reaction product of hydrogen peroxide and a performic acid precursor. Senner et al supra, reported performic acid with a special stabilizer, and hydrogen peroxide with the stabilizer Lufibrol W as being very efficient bleaching agents in the single bath dyeing and bleaching of wool. Arsov et al supra, reported a satisfactory dyeing and bleaching process of wool yarn in which the yarn is first dyed and then a solution containing hydrogen peroxide is added near the end of the dyeing to give bright shades. Nikolova et al supra, reported a single bath dyeing and bleaching process of a 50:50 rayon-wool yarn in which the hydrogen peroxide is added to the dyebath after 20 minutes at the boil to obtain greater color purity and brilliance than yarn processed in the conventional two-bath method. Even though these processes are claimed to give bright shades they are rather limited in the sense that some of the dyes are sensitive to bleaching agents and hence cannot be used. Uehara (Japan ('975)) supra, reports a single bath bleaching and dyeing process of fibers that calls for the bleaching of the fibers in a bath containing urea peroxide as the major component, neutralizing the bath by NaHSO.sub. 3 after bleaching, and dyeing the fibers with the addition of dye to the same bath. Uehara (Japan ('782)) supra, reports a single bath bleaching and dyeing process of textiles, in which the textile is bleached in a bath containing bleaching agent(s), chiefly of peroxide(s), and the agent(s) are decomposed by adding reducing agent, chiefly of thio compounds, to the bath, followed by addition of dye(s) to the bath to effect dyeing. In the example given in the patent, Uehara explains that the bath is drained and replaced by a fresh bath after neutralization of peroxide(s) with reducing agent and then the dyeing is carried out according to conventional methods. Other dyebath bleaching processes that employ products available commercially are summarized in a recent review by Duffield supra.